Combination fire and water tube boiler

ABSTRACT

A natural circulation, shockproof, horizontal furnace, combination water tube and fire tube boiler. The water tubes comprise a plurality of membrane-connected, ring-shaped tubes, defining a substantial portion of the primary combustion cylinder for the burner flame, surrounded by a larger diameter, secondary combustion cylinder which defines the remaining portion of the longitudinal combustion zone, and which redirects the combustion gases in a return path back over the outer surface of the water tube-membrane structure. The secondary combustion cylinder is connected only at one end and is free to expand and contract due to temperature changes without stressing the boiler or the combustion cylinder. The combustion gases then reverse direction a second time in a reversal chamber to pass through horizontal fire tubes extending the length of the boiler directly through the large outer shell containing the water to be heated, and then out through the flue gas outlet. This novel boiler unit can be used to generate steam, or for hot water applications with minor modifications.

United States Patent Craig [54] COMBINATION HERE AND WATER TUBE BOILER [72] Inventor: Glenn 1). Craig, Menomonee Falls, Wis.

[73] Assignee: Aqua-Chem, lnc., Waukesha, Wis.

[22] Filed: Aug. 26, 1969 [21] Appl. No.: 853,023

1 Feb. 1, 1972 Primary Examineri(enneth W. Sprague Att0rneyFred Wiviott and Ralph G. Hohenfeldt [5 7] ABSTRACT A natural circulation, shockproof, horizontal furnace, combination water tube and fire tube boiler. The water tubes comprise a plurality of membrane-connected, ring-shaped tubes, defining a substantial portion of the primary combustion cylinder for the burner flame, surrounded by a larger diameter, secondary combustion cylinder which defines the remaining portion of the longitudinal combustion zone, and which redirects the combustion gases in a return path back over the outer surface of the water tube-membrane structure. The secondary combustion cylinder is connected only at one end and is free to expand and contract due to temperature changes without stressing the boiler or the combustion cylinder. The combustion gases then reverse direction a second time in a reversal chamber to pass through horizontal fire tubes extending the length of the boiler directly through the large outer shell containing the water to be heated, and then out through the flue gas outlet. This novel boiler unit can be used to generate steam, or for hot water applications with minor modifications.

17 Claims, 4 Drawing Figures PATENTEU FEB 1 E72 SHEET 1 BF 2 GLENN D CRAIG INVENTOR ATTORNEY PATENIEB FEB m2 3.638.821

sum 2 or 2 EAT ABSORPTION BTU/HR x |oo,O00

HEAT INPUT BTU/HR X |00,000

FIG. 4 P m. 3 GLENN 9.19m?

BY W M ATTORN EY COMBINATION FIRE AND WATER TUBE BOILER BACKGROUND OF THE INVENTION Many of the available natural circulation boilers are designed for a single application, and efficiency decreases substantially when attempts are made to adapt the same design to other uses. Usually, such a change requires redesign of the boiler components. Typical fire tube boilers tend to have a certain degree of inertia, which makes their use ordinarily undesirable for applications where a rapid recovery rate is required. Water tube boilers, on the other hand, have a much higher recovery rate, but must be carefully controlled to prevent overshoot, when a demand for less steam is made on the system. Fire tube type boilers are subject to thermal shock from rapid temperature changes, when the boiler components expand and contract.

It is desirable to design a universal, natural circulation boiler which has a wide range of operational efiiciency, as well as good application flexibility, and a rapid recovery rate in response to quickly changing steam or hot water requirements.

1. Field of the Invention There is a great need for a shockproof boiler of compact dimensions, which as a flexible response rate and a wide operating efficiency range. There is also a need for a natural circulation system to provide a degree of protection for the boiler where skilled boiler engineers are not available. A single boiler unit is needed which can be modified slightly to produce hot water, low-pressure steam, or high-pressure steam. Such a unit should be compact, shockproof, and is primarily intended for hot water service where rapidly fluctuating temperatures are encountered.

2. Description of the Prior Art Prior water tube boilers incorporate relatively complex water tube arrangements. Some of these incorporate a relatively long, continuous, water tube coil, and it is usually considered necessary to include pump means for forced circulation to protect the heated tubes from burning out. Such an arrangement requires delicate control, and pump failure can necessitate boiler shutdown at an inopportune time.

The typical fire tube, or Scotch marine type boiler, in which combustion takes place in a large, horizontally disposed furnace, usually has two end plates to which the combustion chamber is secured. Combustion gases in the firebox are directed through a plurality of longitudinal fire tubes disposed around the tubular combustion chamber and submerged in a water drum or shell, which surrounds both the combustion chamber and the fire tubes. Expansion and contraction of the combustion chamber due to temperature changes creates a considerable stress of the end plates to which the combustion chamber is secured.

One natural circulation boiler has been proposed which includes water rings or elbows inside a flame tube extending the full length of the boiler body. This flame tube is connected at both ends to the cylindrical body, and this boiler shell is also thereby subjected to thermal shock. There is no means provided to promote turbulent flow of combustion gases around SUMMARY OF THE INVENTION Applicants combination boiler includes the best features of both the water tube boiler, and the Scotch marine fire tube type boiler. Although the ring tube-membrane structure defines a portion of the combustion chamber wall, and is directly exposed to combustion gases on both its inner and outer surfaces, the ring tubes provide an extremely short, natural circulation path for the water being heated, and both the inlets and outlets of the water tubes are submerged below the water level in the shell, so that the possibility of overheating the tubes is negligible. In addition, the ring tube-membrane structure, which defines a primary combustion zone,

confines and redirects the combustion gases along a turbulent flow path for most efficient heat transfer. The ring tube-membrane structure is attached only at the inlet and outlet extensions of the water tubes to the outer, secondary combustion cylinder, which is, in turn, attached only to the end plate at the burner end of the boiler, thereby allowing the opposite, unattached end of the ring tube-membrane structure to contract and expand freely with temperature changes without subjecting the end plates of the furnace to stresses. The outer, secondary combustion cylinder is completely submerged in the water drum, so that the heat transfer efficiency is further improved.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 of the drawings is a perspective view, with parts broken away, of the boiler constructed according to the invention;

FIG. 2 is a diagrammatic, longitudinal section of the apparatus shown in FIG. 1 to clearly illustrate the combustion gas flow path;

FIG. 3 is a transverse section taken on line 3-3 of FIG. 2 to show the natural water circulation through the ring tubes, and showing the relative location of the fire tubes; and

FIG. 4 is a graph comparing the heat absorption of the boiler combustion chamber constructed according to the invention to a straight through combustion chamber.

As seen in the drawings, boiler 1 comprises an outer shell 2, having first and second end walls 3 and 4. The first end wall 3 is enclosed by a refractory combustion gas reversal box 5, which also encloses a refractory ring cone 6, providing an orifice for a jet flame burner 7, supported on an end plate 8.

The second end 4 of the outer shell 2 is enclosed by a flue gas collector box 9, which connects to a flue 10 for discharge of the combustion gases from the system.

A plurality of fire tubes 11 extend longitudinally through the outer shell 2 from the first end wall 3 to the second end wall 4. The fire tubes 11 surround a secondary combustion cylinder 12, which is connected only at its first end 13 to the end wall 3 of the boiler shell 2. Second end 14 of the secondary combustion cylinder 12 is closed and is not attached, so that the cylinder 12 is free to expand and contract inside the boiler shell 2.

A primary combustion cylinder 15 is disposed concentrically inside the secondary combustion cylinder 12, extending only partially along its length. The primary combustion cylinder 15 comprises a plurality of ring-shaped water tubes 16, each having a lower inlet tube 17 and an upper outlet tube 18, and being interconnected by membranes 19 to define a continuous, tubular primary combustion zone for the combustion gases from the jet flame burner 7. The lower inlet tubes 17 and upper outlet tubes 18 are connected to the secondary combustion cylinder 12 to support the primary combustion cylinder therein.

The outer shell 2 is provided with a feed water inlet 20, and a product outlet 21, which are located generally as shown in FIGS. 1 and 2 of the drawings. The product outlet 21 can also be located as shown in FIG. 2 in phantom, depending on the particular application for which the water level 22 in the outer shell 2 is maintained above the level of the fire tubes 11 and the secondary combustion cylinder 12 and all of its internal components.

The thermodynamic, natural circulation, heat transfer functions of the boiler can best be seen by referring to FIGS. 2 and 3. As shown in FIG. 2, the jet burner flame 23, projects into the primary combustion cylinder I5, where the direct contact with the inner surfaces of the ring-shaped water tubes 16 and the interconnecting membranes 19 provides rapid and instantaneous natural circulation of water up through the water tubes 16. The hot combustion gases then enter the secondary combustion cylinder 12, releasing further heat through the walls thereof, and the gases reverse flow at the end wall 14 to then travel over the outer surface of the primary combustion cylinder 15 through the annular space 24 defined between the primary and secondary combustion cylinders 15 and 12. The hot combustion gases then enter the flow reversal box 5 and are directed into the fire tubes 11 to make a final pass through the outer shell 2 to provide maximum heat transfer to the water in the boiler system 1.

The improved heat absorption capacity of the boiler constructed according to the invention is made apparent by the graph of FIG. 4 which compares the heat absorption of a boiler combustion chamber of specific dimensions made according to the invention with a straight through combustion chamber of a typical Scotch marine type boiler of approximately the same dimensions. It can be seen that the percentage increase in absorptivity of the new combustion chamber improves at a substantial rate over that measured for a straight through combustion chamber. Line A of the graph represents the heat absorptivity of a conventional straight through combustion chamber construction. Line B is a plot measuring the heat absorptivity of the new combustion chamber construction, and Line C is a plot (in percentage) showing graphically the improvement in heat absorptivity made possible by the new boiler construction.

For example, a heat input of 5 million B.t.u.s per hour into the straight through combustion chamber provides a heat absorption into the water being heated of about 2 million, two hundred thousand B.t.u.s per hour. The same input into the new combustion chamber provides a heat absorption into the water being heated of about 3 million, three hundred thousand B.t.u.s per hour, an improvement of better than 40 percent.

The practical implications of the improved performance of the new boiler are numerous. For example, a plant utilizing the new boiler can operate at a lower temperature, and requires less additional heat surface for cooling the flue gases to the required gas discharge temperature compared to earlier designs. It is not necessary to include as many fire tubes in the boiler construction to obtain the same boiler performance, thus giving an additional construction economy. Another important benefit flowing from the improved heat absorptivity is a reduction in the overall dimensions of the new boiler for comparable capacity which makes it useful for a number of applications where space is at a premium, such as for shipboard use.

The improved boiler of the invention has wide application, both for hot water and for steam-generating purposes. The natural circulation waterflow, combined with the triple pass combustion gas flow path provides a substantially improved heat absorptivity. The special combustion cylinder construction with the sliding closed end, substantially reduces the possibility of thermal shock caused by uneven expansion and contraction of the combustion cylinder relative to the fire tubes, boiler shell, and the other elements of the boiler.

I claim:

I. A fire tube boiler comprising, an outer horizontal shell having first and second end plates connected thereto, a combustion cylinder disposed horizontally in said outer shell, said combustion cylinder having a first end secured to the first end plate of the outer shell and a second closed end inside said outer shell and said combustion cylinder spaced from and free of said outer shell along the length of said combustion cylinder so that said combustion cylinder is free to expand and contract inside the shell in response to temperature changes without stressing the shell or the combustion cylinder, a plurality of fire tubes disposed around said combustion cylinder and extending longitudinally through said outer shell between the first and second end plates, and conduit means between the first end of the combustion cylinder and the adjacent ends of said fire tubes for directing hot combustion gases from said combustion cylinder through said fire tubes.

2. The apparatus of claim 1, including a smaller cylindrical member disposed within said combustion cylinder and extending through a portion of the length of said combustion cylinder, said cylindrical member including a plurality of generally vertic'al water tubes in the walls of said smaller cylindrical member and exposed directly to the interior of said smaller cylindrical member, and means defining inlets and outlets to said water tubes providing liquid communication of said water tubes with said outer shell.

3. The apparatus of claim 1, including a gas jet burner assembly disposed at the open end of the combustion cylinder, said burner assembly having an annular refractory member defining a generally cone-shaped bumer orifice for guiding the burner flame from the burner assembly horizontally into the combustion cylinder.

4. A fire tube boiler for liquid-phase heating comprising, a boiler shell for holding the liquid phase, said shell having first and second end walls and an elongated, horizontal body portion, a plurality of fire tubes extending horizontally through said boiler shell below the normal liquid phase level, said fire tubes extending between the first and second end walls of the boiler shell, an elongated combustion cylinder disposed horizontally in the boiler shell and secured only to the first end wall of the boiler shell, said combustion cylinder having a second closed end disposed inside the body portion and spaced from the second end wall of the boiler shell and being free along its extension between said open and closed ends so that said combustion cylinder is free to expand and contract inside the boiler shell in response to temperature changes without stressing the boiler shell or the combustion cylinder, and means disposed in the area of said open end of said combustion cylinder for directing the combustion gases to said fire tubes after the combustion gases have been reversed in flow direction by the second closed end of the combustion cylinder.

5. The apparatus of claim 4, including an elongated tubular structure disposed inside the first open end of said combustion cylinder, said tubular structure arranged generally concentrically within the combustion cylinder to define a primary combustion zone therein and defining an annular return combustion gas flow path between the adjacent surfaces of the tubular structure and the combustion cylinder to ensure combustion gas flow through the combustion cylinder.

6. The apparatus of claim 5, in which the elongated tubular structure comprises a plurality of ring-shaped liquid phase tubes directly exposed to the interior of said tubular structure each having upper and lower tubular extensions providing natural liquid circulation in the boiler shell through the ringshaped liquid phase tubes when the boiler is operative, and burner means directed into said tubular structure.

7. A boiler including a horizontally disposed cylindrical boiler shell having first and second end walls, a combustion cylinder having a first open end connected to said first end wall and extending into said boiler shell from said first end wall terminating in a closed end, means at the first end of said combustion cylinder defining a burner flame orifice and combustion gas outlet, said second closed end of the combustion cylinder being disposed in the boiler shell at a sufficient distance from the second end wall to allow stress-free expansion and contraction of the combustion cylinder and the boiler shell relative to each other.

8. A combination water tube, fire tube boiler comprising, in combination, a horizontally disposed cylindrical boiler shell and a heat input cylinder disposed horizontally therein, a natural circulation, water tube heater assembly including a plurality of ring-shaped water tubes disposed generally concentrically inside a portion of the heat input cylinder, said water tubes having upper and lower extensions secured to the top and bottom surfaces of the heat input cylinder to provide waterflow upwardly through the water tubes, membrane means interconnecting the ring-shaped water tubes to define a primary heat input zone surrounded by the water tube heater assembly, a secondary heat input zone being defined by the remaining portion of the heat input cylinder and the outer surface of the water tube heater assembly, heat input means for first transferring heat energy in the primary heat input zone and then transferring heat energy into the water in the secondary heat input zone, and gas flow means for transferring heat energy remaining after passage through the primary and secondary heat input zones including a plurality of horizontally disposed fire tubes surrounding the heat input cylinder and means for transferring said remaining heat energy to said fire tubes from said secondary heat input zone whereby optimum heat energy transfer to the water being heated is realized.

9. The apparatus of claim 8, in which the heat input cylinder has an open end and a closed end and is attached to the boiler only at the open end to permit relative movement between the heat input cylinder closed end and the boiler shell, whereby thermal stresses are avoided.

10. The apparatus of claim 8, in which the gas flow means includes an annular refractory gas flow reversal box for redirecting combustion gases from the secondary heat input zone into the horizontally disposed fire tubes.

11. The apparatus of claim 8, in which the heat input cylinder is connected to the boiler shell at one end and slidably supported within the boiler shell to provide a thermal shockproof mounting means for said heat input cylinder.

12. The apparatus of claim 6, wherein said liquid-phase tubes are also directly exposed to the annular return combustion gas flow path.

13. A combination water tube, fire tube boiler comprising, in combination, a boiler shell, a generally horizontal combustion cylinder within said shell including a plurality of water tubes defining a primary combustion region which is spaced from the interior surface of said combustion cylinder with corresponding sides of the water tubes presented toward the interior of said combustion cylinder through said primary combustion region, means defining inlets and outlets in said water tubes for liquid circulation through said water tubes within said boiler, a plurality of fire tubes disposed generally below the normal liquid level in said boiler, and means for directing combustion gases from said combustion cylinder to said fire tubes.

14. A fire tube boiler comprising, an outer horizontal shell having first and second end plates connected thereto, a combustion cylinder disposed horizontally in said outer shell, said combustion cylinder having a first end secured to the first end plate of the outer shell and a second closed end supported inside said outer shell for longitudinal movement relative to the second end plate, a plurality of fire tubes disposed around said combustion cylinder and extending longitudinally through said outer shell between the first and second end plates, conduit means between the first end of the combustion cylinder and the adjacent ends of said fire tubes for directing hot combustion gases from said combustion cylinder through said fire tubes, a smaller cylindrical member disposed inside the combustion cylinder and connected to the first end thereof, said cylindrical member extending through a portion of the length of the combustion cylinder and defining an annular gas passage between the adjacent surfaces of said combustion cylinder and said cylindrical member, and said cylindrical member comprising a plurality of vertically disposed, ringshaped water tubes, each having lower inlet and upper outlet tubes extending through the combustion cylinder and supporting the smaller cylindrical member therein and a plurality of membranes interconnecting the ring-shaped water tubes to define a continuous cylindrical member having both inner and outer surfaces exposed to combustion gases and having the inlets and outlets of the ring-shaped water tubes in liquid communication with said outer shell.

15. A fire tube boiler for liquid-phase heating comprising, a boiler shell for holding the liquid phase, said shell having first and second end walls and an elongated, horizontal body portion, a plurality of fire tubes extending horizontally through said boiler shell below the normal liquid-phase level, said fire tubes extending between the first and second end walls of the boiler shell, an elongated combustion cylinder disposed horizontally in the boiler shell and secured only to the first end wall of the boiler shell, said combustion cylinder having an 0 en end at the first end wall of the boiler shell and having a c osed end disposed inside the body portion and spaced from the second end wall of the boiler shell, said combustion cylinder being free to expand and contract inside the boiler shell in response to temperature changes without stressing the boiler shell or the combustion cylinder, means disposed in the first open end of said combustion cylinder dividing said first open end into a burner orifice for entry of the burner flame into the combustion cylinder and an outlet for exit of the combustion gases from the first end of the combustion cylinder after the combustion gases have been reversed in flow direction by the second closed end of the combustion cylinder, an elongated tubular structure disposed inside said combustion cylinder, said tubular structure extending generally concentrically into the combustion cylinder to define a primary combustion zone therein and also defining an annular return combustion gas flow path between the adjacent surfaces of the tubular structure and the combustion cylinder to ensure turbulent combustion gas flow through the combustion cylinder, the elongated tubular structure including a plurality of ring-shaped liquid phase tubes each having upper and lower tubular extensions extending through the combustion cylinder walls and supporting the tubular structure in spaced relationship to the combustion cylinder, said tubular extensions also providing natural liquid circulation in the boiler shell through the ring-shaped liquid phase tubes when the boiler is operative.

16. A fire tube boiler comprising:

a. means defining a space for water,

b. combustion chamber means extending into said water space with its extema] surface exposed thereto, said chamber means being closed within said water space and having a closed end and also having a first end with an opening remote from said closed end,

c. means directing hot combustion gases from said first end of said chamber means toward said closed end, said gases returning toward said first end along the internal surface of said chamber,

d. a plurality of fire tubes having corresponding ends adjacent the first end of the chamber means and extending through said water space in spaced relation with said combustion chamber means, and

e. conduit means adapted to direct said returning combustion gases from said-opening in the first end of said chamber means into the adjacent ends of said fire tubes.

17. The invention set forth in claim 16 including:

a. water-conducting means disposed inside of said combustion chamber means near its said first end, said conducting means having intemal and external wall surfaces, said internal surface defining a heat exchange surface and a passageway for hot gases moving away from said first end toward said closed end and said external surface being spaced from said combustion chamber means to define a heat exchange surface and a passageway for gases returning toward said first end,

b. said water-conducting means having inlet and outlet conduit means extending in sealing relation through said combustion chamber means and communicating with said space for water. 

1. A fire tube boiler comprising, an outer horizontal shell having first and second end plates connected thereto, a combustion cylinder disposed horizontally in said outer shell, said combustion cylinder having a first end secured to the first end plate of the outer shell and a second closed end inside said outer shell and said combustion cylinder spaced from and free of said outer shell along the length of said combustion cylinder so that said combustion cylinder is free to expand and contract inside the shell in response to temperature changes without stressing the shell or the combustion cylinder, a plurality of fire tubes disposed around said combustion cylinder and extending longitudinally through said outer shell between the first and second end plates, and conduit means between the first end of the combustion cylinder and the adjacent ends of said fire tubes for directing hot combustion gases from said combustion cylinder through said fire tubes.
 2. The apparatus of claim 1, including a smaller cylindrical member disposed within said combustion cylinder and extending through a portion of the length of said combustion cylinder, said cylindrical member including a plurality of generally vertical water tubes in the walls of said smaller cylindrical member and exposed directly to the interior of said smaller cylindrical member, and means defining inlets and outlets to said water tubes providing liquid communication of said water tubes with said outer shell.
 3. The apparatus of claim 1, including a gas jet burner assembly disposed at the open end of the combustion cylinder, said burner assembly having an annular refractory member defining a generally cone-shaped burner orifice for guiding the burner flame from the burner assembly horizontally into the combustion cylinder.
 4. A fire tube boiler for liquid-phase heating comprising, a boiler shell for holding the liquid phase, said shell having first and second end walls and an elongated, horizontal body portion, a plurality of fire tubes extending horizontally through said boiler shell below the normal liquid phase level, said fire tubes extending between the first and second end walls of the boiler shell, an elongated combustion cylinder disposed horizontally in the boiler shell and secured only to the first end wall of the boiler shell, said combustion cylinder having a second closed end disposed inside the body portion and spaced from the second end wall of the boiler shell and being free along its extension between said open and closed ends so that said combustion cylinder is free to expand and contract inside the boiler shell in response to temperature changes without stressing the boiler shell or the combustion cylinder, and means disposed in the area of said open end of said combustion cylinder for directing the combustion gases to said fire tubes after the combustion gases have been reversed in flow direction by the second closed end of the combustion cylinder.
 5. The apparatus of claim 4, including an elongated tubular structure disposed inside the first open end of said combustion cylinder, said tubular structure arranged generally concentrically within the combustion cylinder to define a primary combustion zone therein and defining an annular return combustion gas flow path betweeN the adjacent surfaces of the tubular structure and the combustion cylinder to ensure combustion gas flow through the combustion cylinder.
 6. The apparatus of claim 5, in which the elongated tubular structure comprises a plurality of ring-shaped liquid phase tubes directly exposed to the interior of said tubular structure each having upper and lower tubular extensions providing natural liquid circulation in the boiler shell through the ring-shaped liquid phase tubes when the boiler is operative, and burner means directed into said tubular structure.
 7. A boiler including a horizontally disposed cylindrical boiler shell having first and second end walls, a combustion cylinder having a first open end connected to said first end wall and extending into said boiler shell from said first end wall terminating in a closed end, means at the first end of said combustion cylinder defining a burner flame orifice and combustion gas outlet, said second closed end of the combustion cylinder being disposed in the boiler shell at a sufficient distance from the second end wall to allow stress-free expansion and contraction of the combustion cylinder and the boiler shell relative to each other.
 8. A combination water tube, fire tube boiler comprising, in combination, a horizontally disposed cylindrical boiler shell and a heat input cylinder disposed horizontally therein, a natural circulation, water tube heater assembly including a plurality of ring-shaped water tubes disposed generally concentrically inside a portion of the heat input cylinder, said water tubes having upper and lower extensions secured to the top and bottom surfaces of the heat input cylinder to provide waterflow upwardly through the water tubes, membrane means interconnecting the ring-shaped water tubes to define a primary heat input zone surrounded by the water tube heater assembly, a secondary heat input zone being defined by the remaining portion of the heat input cylinder and the outer surface of the water tube heater assembly, heat input means for first transferring heat energy in the primary heat input zone and then transferring heat energy into the water in the secondary heat input zone, and gas flow means for transferring heat energy remaining after passage through the primary and secondary heat input zones including a plurality of horizontally disposed fire tubes surrounding the heat input cylinder and means for transferring said remaining heat energy to said fire tubes from said secondary heat input zone whereby optimum heat energy transfer to the water being heated is realized.
 9. The apparatus of claim 8, in which the heat input cylinder has an open end and a closed end and is attached to the boiler only at the open end to permit relative movement between the heat input cylinder closed end and the boiler shell, whereby thermal stresses are avoided.
 10. The apparatus of claim 8, in which the gas flow means includes an annular refractory gas flow reversal box for redirecting combustion gases from the secondary heat input zone into the horizontally disposed fire tubes.
 11. The apparatus of claim 8, in which the heat input cylinder is connected to the boiler shell at one end and slidably supported within the boiler shell to provide a thermal shockproof mounting means for said heat input cylinder.
 12. The apparatus of claim 6, wherein said liquid-phase tubes are also directly exposed to the annular return combustion gas flow path.
 13. A combination water tube, fire tube boiler comprising, in combination, a boiler shell, a generally horizontal combustion cylinder within said shell including a plurality of water tubes defining a primary combustion region which is spaced from the interior surface of said combustion cylinder with corresponding sides of the water tubes presented toward the interior of said combustion cylinder through said primary combustion region, means defining inlets and outlets in said water tubes for liquid circulation through said water tubes within said boiler, a plurality of fire tubes disposed generally below the normal liquid level in said boiler, and means for directing combustion gases from said combustion cylinder to said fire tubes.
 14. A fire tube boiler comprising, an outer horizontal shell having first and second end plates connected thereto, a combustion cylinder disposed horizontally in said outer shell, said combustion cylinder having a first end secured to the first end plate of the outer shell and a second closed end supported inside said outer shell for longitudinal movement relative to the second end plate, a plurality of fire tubes disposed around said combustion cylinder and extending longitudinally through said outer shell between the first and second end plates, conduit means between the first end of the combustion cylinder and the adjacent ends of said fire tubes for directing hot combustion gases from said combustion cylinder through said fire tubes, a smaller cylindrical member disposed inside the combustion cylinder and connected to the first end thereof, said cylindrical member extending through a portion of the length of the combustion cylinder and defining an annular gas passage between the adjacent surfaces of said combustion cylinder and said cylindrical member, and said cylindrical member comprising a plurality of vertically disposed, ring-shaped water tubes, each having lower inlet and upper outlet tubes extending through the combustion cylinder and supporting the smaller cylindrical member therein and a plurality of membranes interconnecting the ring-shaped water tubes to define a continuous cylindrical member having both inner and outer surfaces exposed to combustion gases and having the inlets and outlets of the ring-shaped water tubes in liquid communication with said outer shell.
 15. A fire tube boiler for liquid-phase heating comprising, a boiler shell for holding the liquid phase, said shell having first and second end walls and an elongated, horizontal body portion, a plurality of fire tubes extending horizontally through said boiler shell below the normal liquid-phase level, said fire tubes extending between the first and second end walls of the boiler shell, an elongated combustion cylinder disposed horizontally in the boiler shell and secured only to the first end wall of the boiler shell, said combustion cylinder having an open end at the first end wall of the boiler shell and having a closed end disposed inside the body portion and spaced from the second end wall of the boiler shell, said combustion cylinder being free to expand and contract inside the boiler shell in response to temperature changes without stressing the boiler shell or the combustion cylinder, means disposed in the first open end of said combustion cylinder dividing said first open end into a burner orifice for entry of the burner flame into the combustion cylinder and an outlet for exit of the combustion gases from the first end of the combustion cylinder after the combustion gases have been reversed in flow direction by the second closed end of the combustion cylinder, an elongated tubular structure disposed inside said combustion cylinder, said tubular structure extending generally concentrically into the combustion cylinder to define a primary combustion zone therein and also defining an annular return combustion gas flow path between the adjacent surfaces of the tubular structure and the combustion cylinder to ensure turbulent combustion gas flow through the combustion cylinder, the elongated tubular structure including a plurality of ring-shaped liquid phase tubes each having upper and lower tubular extensions extending through the combustion cylinder walls and supporting the tubular structure in spaced relationship to the combustion cylinder, said tubular extensions also providing natural liquid circulation in the boiler shell through the ring-shaped liquid phase tubes when the boiler is operative.
 16. A fire tube boiler comprising: a. means defining a space for water, b. combustion chamber mEans extending into said water space with its external surface exposed thereto, said chamber means being closed within said water space and having a closed end and also having a first end with an opening remote from said closed end, c. means directing hot combustion gases from said first end of said chamber means toward said closed end, said gases returning toward said first end along the internal surface of said chamber, d. a plurality of fire tubes having corresponding ends adjacent the first end of the chamber means and extending through said water space in spaced relation with said combustion chamber means, and e. conduit means adapted to direct said returning combustion gases from said opening in the first end of said chamber means into the adjacent ends of said fire tubes.
 17. The invention set forth in claim 16 including: a. water-conducting means disposed inside of said combustion chamber means near its said first end, said conducting means having internal and external wall surfaces, said internal surface defining a heat exchange surface and a passageway for hot gases moving away from said first end toward said closed end and said external surface being spaced from said combustion chamber means to define a heat exchange surface and a passageway for gases returning toward said first end, b. said water-conducting means having inlet and outlet conduit means extending in sealing relation through said combustion chamber means and communicating with said space for water. 